Forced air furnaces for heating glass sheets in preparation for subsequent processing, such as tempering, are known in the art. For example, McMaster, U.S. Pat. Nos. 4,529,380 and 4,505,671, discloses a glass sheet processing system which includes a heating furnace and a processing station for processing heated glass sheets to provide bending, tempering, bending and tempering, filming, etc. The furnace of U.S. Pat. No. 4,592,380 and 4,505,671 comprises an array of gas jets spaced above a conveyor within a heating chamber. The gas jets supply a primary gas flow directed toward the conveyor to provide forced convection heating of the glass sheets as the sheets are conveyed through the heating chamber.
The gas jets of McMaster are arranged in linear series perpendicular to the length of the conveyor and the direction of travel of the glass sheets. Each series of jets is connected to a common linear supply manifold or conduit. Each supply conduit also extends widthwise in the heating furnace, perpendicular to the length of the conveyor. McMaster teaches that the array of gas jet pumps are spaced from each other transversely to the direction of conveyance so as to uniformly heat each conveyed glass sheet over its entire width.
Heating systems such as described by McMaster appear to provide acceptable results for heating clear glass prior to tempering. Other known systems provide acceptable results for heating coated glass having an emissivity rating greater than about 0.2 prior to tempering. However, manufacturers have now begun to produce coated glass products having emissivity ratings in the range of 0.15-0.04. Prior art heating systems, including the system disclosed in U.S. Pat. Nos. 4,592,380 and 4,505,671, do not provide acceptable results for tempering glass having such low emissivity ratings. Therefore, it would be desirable to provide a system and method of tempering low "e" glass sheeting having an emissivity rating below 0.2.
When glass sheets are conveyed into a heating furnace, the bottom surface heats at a faster rate than the top surface due to contact with the rolls of the conveyor. This causes the bottom surface to expand at a faster rate than the top surface which results in the glass bowing upward into the shape of a bowl. All of the glass sheet's weight is supported in the center of the glass which causes the center of the glass to be overheated. This results in excessive distortion in the center of the glass which can be described as an elongated bubble. Non-uniform glass temperatures also cause the glass to oil can (or become bi-stable). Oil canning (or by-stability) and bubbling are undesirable conditions produced in the glass when the glass sheeting is not heated uniformly. Therefore, it would also be desirable to provide a method and apparatus for heating low "e" coated glass sheets which minimizes oil-canning and bubbling.
When low "e" glass is tempered in prior art systems, it is typically run lengthwise through the furnace due to the size of the furnace. It is also run lengthwise to mitigate the appearance of inherent distortions because the glass sheets are typically installed lengthwise down a room or hallway. However, low emissivity glass is more sensitive to heating in the longitudinal direction than it is in the widthwise direction. When glass is tempered in prior art systems, heat is only applied uniformly over the width of the glass sheet. This does not allow for separate control from edge to edge across the width of the glass. Without this control, the glass will not be heated as uniformly and the undesirable conditions of center bubble and oil canning will ensue. Therefore, it is also desirable to provide a system and method of tempering which uniformly applies heat over the entire length of the sheet in a longitudinal direction to improve aesthetic quality.